Java: Check if command line arguments are null

I am looking to do some error checking for my command line arguments

public static void main(String[] args)
{
    if(args[0] == null)
    {
        System.out.println("Proper Usage is: java program filename");
        System.exit(0);
    }
}

However, this returns an array out of bounds exception, which makes sense. I am just looking for the proper usage.

Solution 1:

The arguments can never be null. They just wont exist.

In other words, what you need to do is check the length of your arguments.

public static void main(String[] args)
{
    // Check how many arguments were passed in
    if(args.length == 0)
    {
        System.out.println("Proper Usage is: java program filename");
        System.exit(0);
    }
}

Solution 2:

@jjnguy's answer is correct in most circumstances. You won't ever see a null String in the argument array (or a null array) if main is called by running the application is run from the command line in the normal way.

However, if some other part of the application calls a main method, it is conceivable that it might pass a null argument or null argument array.

However(2), this is clearly a highly unusual use-case, and it is an egregious violation of the implied contract for a main entry-point method. Therefore, I don't think you should bother checking for null argument values in main. In the unlikely event that they do occur, it is acceptable for the calling code to get a NullPointerException. After all, it is a bug in the caller to violate the contract.

Solution 3:

To expand upon this point:

It is possible that the args variable itself will be null, but not via normal execution. Normal execution will use java.exe as the entry point from the command line. However, I have seen some programs that use compiled C++ code with JNI to use the jvm.dll, bypassing the java.exe entirely. In this case, it is possible to pass NULL to the main method, in which case args will be null.

I recommend always checking if ((args == null) || (args.length == 0)), or if ((args != null) && (args.length > 0)) depending on your need.